The present invention relates to a spatial and temporal equalizer that is a hybrid of an adaptive array antenna and an adaptive equalizer for mobile communications, and a spatial and temporal equalization method.
The adaptive array antenna and the adaptive equalizer are effective in removing interference in mobile communications. The adaptive array antenna adaptively generates a beam pattern (a directional pattern) in which a beam of a relatively high antenna gain (the main lobe of the directional pattern) is directed toward the wave desired to receive and null of the directional pattern, for which the antenna gain is significantly low, are directed toward interference waves such as signals from other users. The adaptive array antenna performs spatial signal processing and is an effective means for eliminating interference waves of the same channel as that of the desired waves, that is, cochannel interference.
The adaptive equalizer has been used to eliminate multipath waves that are desired waves but delayed behind them, that is, intersymbol interference. The spatial and temporal equalizer is a combination of the adaptive array antenna and the adaptive equalizer.
In FIG. 10 there is depicted a conventional spatial and temporal equalizer disclosed, for example, in Saito et al., “A Study of a split channel estimation scheme for the Spatial and Temporal Equalizer,” Technical Report of IEICE, DSP99-178, SAT99-133, RCS99-183 (2000-01), pp. 25-30 (hereinafter referred to as Literature 1), and Fukawa, “A cascading Connection of Adaptive Array and MLSE Detector and its Performances,” Technical Report of IEICE, A-p97-146 (1997-11), pp. 85-92 (hereinafter referred to as Literature 2). In this prior art example, respective elements A1, A2, . . . , AL of an adaptive antenna 10 are equipped with feed forward filters F1, F2, . . . , FL for compensating for symbol timing offset. Symbol timing offset from the received signal will often degrade the characteristics of an adaptive equalizer 11, but this can be avoided by the provision of the feed forward filters F1 to FL. It is necessary that taps of a transversal filter forming each of the feed forward filters F1 to FL be set at shorter time intervals than the transmission symbol period T, usually at T/2 time intervals. The outputs from the feed forward filters F1 to FL are combined by a combiner 12, thereafter being fed to the adaptive equalizer 11. In the illustrated spatial and temporal equalizer, tap coefficients of the feed forward filters F1 to FL connected to the antenna elements A1 to AL of an adaptive array 15A and the adaptive equalizer 11 are all simultaneously calculated and set by tap coefficient calculating part 13. Accordingly, the tap coefficients can be converged to optimum values as a whole. To perform this, it is customary to use what is called a training signal for which the transmission symbol pattern is known at the receiving side. With the configuration of FIG. 10, the total number of taps of the feed forward filters F1 to FL is so large that the computational complexity for the tap coefficient calculation by the tap coefficient calculating part 13 increases, giving rise to the problem of extended time of convergence of the tap coefficients.
Literature 1 also discloses a simplified version of the spatial and temporal equalizer of FIG. 10. FIG. 11 depicts such a simplified configuration that is intended to reduce the total number of taps used. As shown in FIG. 11, in this equalizer the feed forward filters F1 to FL are not provided, but instead the outputs from the antenna elements F1 to FL are multiplied by weights (tap coefficients) by multipliers M1 to ML and the multiplied outputs are combined by the combiner 12 and provided via one feed forward filter 14 to the adaptive equalizer 11. That is, the feed forward filter 14 is provided between the combiner 12 and the adaptive equalizer 11, and the multipliers M1 to ML are connected to the antenna elements A1 to AL to complex-multiply their outputs by weights (tap coefficients) to control the phases and amplitudes of received signals. With this arrangement, however, in the case of estimating all the tap coefficients at the same time, no sufficient convergence can be achieved since the tap coefficients for the multipliers M1 to ML of the adaptive array antenna 10 and the tap coefficients of the feed forward filter 14 are provided in the form of product. Accordingly, the tap coefficients are calculated separately.
To start with, the weights (tap coefficients) for the adaptive array antenna 10 and the tap coefficients of the adaptive equalizer are simultaneously converged by tap coefficient calculating part 16 using the first half period of the training signal. In this case, tap coefficients of the feed forward filter 14 are set by tap coefficient calculating part 17 so that the transfer function of the filter 14 is 1, that is, the filter 14 simply passes signals. Next, the tap coefficients of the feed forward filter 14 and the adaptive equalizer 11 are simultaneously converged by the tap coefficient calculating part 17 using the second half period of the training signal. Accordingly, the tap coefficients are set in the adaptive equalizer 11 by the tap coefficient calculating part 16 or 17 that are switched by a switch 18 as required.
In either of the configurations of FIGS. 10 and 11, the tap coefficients are calculated so that the adaptive array antenna 10 eliminates spatially different interference waves of other users and long-delayed waves that the adaptive equalizer 11 cannot equalize and the adaptive equalizer 11 processes short-delayed waves which are equalizable in the time domain, such as intersymbol interference.
In the FIG. 10 example for batch processing as mentioned above, since each element of the adaptive array antenna 10 has the feed forward filter FFF, an increase in the number of antenna elements causes a dramatic increase in the total number of taps, and computational complexity increases accordingly. This constitutes a serious obstacle to forming the spatial and temporal equalizer by hardware, and hence makes it impossible to implement the equalizer. Further, since the convergence of the tap coefficients requires a long training signal period, the transmission efficiency is appreciably low.
With the scheme of FIG. 11, separate convergence of the tap coefficients of the adaptive array antenna 10, the feed forward filter 14 and the adaptive equalizer 11 does not always provide optimum convergence results, and the receiving characteristic is lower than in the case of batch processing. Besides, since the feed forward filter 14 is in the simply-pass state during the tap coefficient convergence of the adaptive array antenna 10, no sufficient convergence can be achieved if symbol timing offset occurs.